It seems the consensus is that over-the-ear noise-cancelling headphones are preferred over earbuds, for a multiplicity of reasons.
I don't just use my headphones on planes, I use them at home as well. I have a little widget that converts SP/DIF Dolby 5.1 or dts digital audio into binaural sound for headphones; you only have two ears, so why do you need 5.1 speakers? For me, I want real audio quality, and, again my personal choice, I'd go for headphones made by a hi-fi audio manufacturer, such as Bowers & Wilkins, as recommended to me by fellow PPRuNers in this sub-forum. For those at the pointy end, comfort may be more of a priority on long flights. I've found the B&W perfectly comfortable, but then I don't have to wear them continuously for a 14 hour flight. They are quite heavy (big batteries!) and the noise cancellation requires a good seal over the ears, I imagine this latter is true for all over-the-ear headphones.